1. Field of the Invention
The present invention pertains to a drilling assembly and method wherein the drill bit, a bit-drive motor and measurement-while-drilling or logging-while-drilling instruments are retrievable through the drillstem and the drillstem itself may be left in the wellbore to serve as the casing or wellbore liner.
2. Background
Conventional rotary drilling operations require relatively frequent withdrawal of the elongated sectionalized drillstem or "drillstring" from the wellbore to inspect or replace the drill bit or portions of the drillstem, to perform well logging operations and to install permanent well casing. This insertion and withdrawal process is time-consuming, hazardous to operating personnel and increases the possibility of damaging the well due to inadvertent dropping of the drillstring into the wellbore or encountering the influx of formation fluids into the wellbore due to the swabbing effect encountered during the drillstring insertion and removal process.
To overcome these problems and hazards, certain techniques have been proposed for drilling with retrievable bits and with drillstring arrangements wherein a liner or casing is inserted into the wellbore coextensively with the drillstem and drilling bit. British Published Patent Application 2,216,926A to Jumblefierce Limited describes a lining or casing assembly which is advanced through the wellbore which is being formed independently by a drill bit connected to a drillstem which extends through the casing and is advanced simultaneously with the casing. U.S. Pat. No. 4,651,837 to W. G. Mayfield describes a retrievable drill bit which may be inserted in and retrieved through a drillstem. However, the simultaneous advance of both the casing and drillstem has certain shortcomings with respect to complications in making up the joints between the drillstem sections and the casing sections, the added weight of the double stem and other complications of using double-stem components. Accordingly, the objective remains to significantly reduce the cost of drilling and installing casing when using any one of known drilling fluid mediums for drill cutting removal and the like, which is at least one of the objectives met by the present invention.